1. Field of the Invention
The present invention relates generally to non-metallic honeycomb structures which are heat formed (thermoformed) into a variety of different shapes. More particularly, the present invention is directed to such structures which include a resin impregnated composite core which is coated with one or more matrix resins to improve the structural strength of the honeycomb.
2. Description of Related Art
Honeycomb structures are well known and widely used in many applications where a high strength and lightweight material is required. The combined features of lightweight and strength found in honeycomb structures makes them particularly well-suited for use in aircraft and other applications where high strength and low weight are required. Honeycomb structures have been made from a wide variety of materials including metals, such as aluminum. Composite materials made from resin impregnated fibers and papers have also been widely used in honeycomb structures. Thermosetting resins have typically been used as the resin matrix of choice in cases where the honeycomb is used as part of a structural member where optimum honeycomb strength is required.
One common process for fabricating honeycomb structures involves bonding multiple sheets of material together along specially oriented node lines. The node lines are offset between different layers in such a way that a honeycomb structure is formed when the layers are expanded. This type of process is commonly referred to as "expansion" process. The expansion process is not suitable for fabricating honeycomb structures in some instances where certain thermoset matrix resins are used. For example, sheets made from certain materials which are too stiff or porous cannot be formed into honeycomb structures using the expansion process.
A fabrication process or method commonly referred to as the "corrugation" process has been used to form high strength thermoset honeycomb structures in those situations where the expansion process cannot be used. The corrugation process involves initially shaping sheets of uncured thermoset or thermoplastic material into a corrugated configuration. The corrugated sheets are cured at high temperature to form stiff corrugated sheets which are then bonded together to form the honeycomb core. The honeycomb core is then optionally coated with a matrix or dip resin. The honeycomb core is generally cut into numerous flat panels which can be used "as is" or further processed in accordance with conventional honeycomb fabrication techniques. For example, the honeycomb core may be sandwiched between sheets of various materials to form extremely strong structural panels.
In many instances, it is desirable to take the high strength thermoset honeycomb core and shape it into non-planar structural elements. This is accomplished by heating the honeycomb core until it becomes sufficiently soft to allow it to be molded or otherwise shaped into the desired configuration. A number of problems have been experienced during the heat forming of thermoset panels into non-planar shapes. For example, the final structural strength of thermoset honeycomb cores can be adversely affected by the high temperatures required to soften the cured resin. In addition, many of the high strength adhesives that are used to bond the panels together become weak at temperatures below those required to soften the thermoset material. As a result, the cores become uncontrollably distorted and weakened during the heat forming process. This problem is especially acute for heavy density and/or small cell honeycomb cores.
In view of the above, there is a need to provide honeycomb cores which are sufficiently strong to be useful as structural panels and which can be heat molded into non-planar shapes without unduly distorting or otherwise adversely affecting the honeycomb structure or strength.